Sunday, May 1, 2005
From The Left Seat
Settling With Power
By Johan Nurmi
At an air show, a huge military helicopter hovered at 100 ft., demonstrating its superb qualities. The crowd of thousands was astounded. Suddenly the aircraft seemed to lose lift and fell from the sky. It was totaled. The pilots walked away but didn't look happy. They knew what had happened, but were not able to correct for it in time.
Settling with power is a dangerous condition in which a helicopter settles in its own downwash, or in its own power. There are four conditions involved:
- Tailwind--In the "weather cock region," with wind entering the tail rotor from about 120-240 deg.
- Airspeed--Below effective translational lift (30 kt. or less).
- Rate of descent--More than 300 fpm.
- Engine power--At more than 20 percent. (Some engine power must be present.)
Settling with power may occur if you, while practicing quick stop at altitude, flare the helicopter to a full stop in a tailwind and allow it to descend vertically. The descent will increase to up to 5,000 fpm if you do not correct. If there's not enough altitude, you may not have time to correct.
During settling with power, you will experience severe vibration and pitch, roll and yaw, plus the descent rate. If you don't act immediately, you will lose control of the helicopter. Pilots performing hover practice with 360-deg. pedal turns at altitude will encounter tailwind at some point, with the risk of getting into settling with power if they allow a descent.
At altitude and below 53 kt. airspeed (the airspeed for best rate of climb, best glide in autorotation and lowest drag in the R22), you are operating on the back side of the power curve. Here your helicopter requires more power to go slower and less to go faster. If hovering and practicing pedal turns at altitude with a heavy gross weight, the aircraft might be able to hover into the wind (depending on temperature, density altitude and wind speed). But as the pilot makes a pedal turn into a tailwind, the power required increases. Since the pilot already might be at or near max power, there may not be enough power available to sustain the hover. As a result, the aircraft may start to descend. This is the perfect setup for settling with power.
Students have had settling-with-power accidents when they did a steep approach with too little forward speed while landing in a tailwind, especially when they are overshooting the landing zone. In this case, the student may perform a harsh flare and enter settling with power almost immediately.
Pilots have entered settling with power while landing to confined areas or shooting a steep approach to a pinnacle, unaware of their slow forward airspeed and high descent rate. That is all it takes to crash a helicopter.
Out in the desert or the back country, it can be difficult to find wind direction. But if you hover at 8 ft. and make a 360-deg. pedal turn, you can determine wind direction by noting when engine power is lowest. That is an indication that you are pointed into the wind.
Many helicopter accidents have been caused by pilots decreasing airspeed close to zero during approach before decreasing the descent rate. When the pilot lifts the collective and flares, he does so into his own downwash, increasing power and pitch angle on the blades. The helicopter settles in its own power and begins to enter the vortex ring state. In most such cases, an accident results.
Settling with power is dangerous because it involves an escalating descent rate and its recovery most often requires even more altitude loss.
Imagine hovering at altitude, in a tailwind, at a high power setting. If you lower the collective and start to descend at more than 300 fpm., it doesn't take too much to bring on vortex ring state. The inner and outer sections of each blade produce less rotor thrust and the rate of descent increases. If you further lower the collective, the blades' root sections go deeper into stall and the tip sections further lose lift. The only section producing effective thrust is a small area between the root and tip. This is not enough to support your weight, and the descent will increase more. In seconds, it may exceed 2,000 fpm.
If you do not understand what is going on, you probably will pull collective, and the accident will be set.
If you are caught in settling with power, your main focus should be to remove the helicopter from the airflow condition that is causing the problem. Rotor-disc attitude must be changed and airspeed increased so the induced flow is no longer opposed to the same extent by the descent-rate flow.
Lower collective slightly to decrease angle of attack and pitch angle. Apply slightly forward cyclic to gain effective translational lift (more than 30 kt. airspeed). Control excessive yaw, roll and pitch with a steady hand on the cyclic.Once you've built up forward airspeed, lift collective again to regain altitude.
Recovery also can be achieved by doing a steady pedal turn into the wind. As soon as you are headed into the wind, the descent rate should decrease.
But you still want to increase forward airspeed to 30 kt. or more for safety. There are days when there is little lift in the air--usually when winds are calm, density altitude is more than 3,000 ft., temperature is above normal and the helicopter is close to max gross weight. These are days when many training accidents occur.
You can also recover by entering autorotation. After entry, you have left the settling-with-power condition and you may roll on rpm again. When the needles have correlated, you may lift collective and regain your loss of altitude.
Don't practice settling with power without an experienced instructor pilot.